The prior art includes an application of D. Sievenpiper, E. Yablonovitch, “Circuit and Method for Eliminating Surface Currents on Metals” U.S. provisional patent application, Ser. No. 60/079,953, filed on Mar. 30, 1998 which relates to a high-impedance or Hi-Z surface and its corresponding PCT application PCT/US99/06884, published as WO99/50929 on Oct. 7, 1999 which application discloses a high impedance surface (also called a Hi-Z or a Frequency Selective Surface herein).
The Hi-Z surface, which is the subject matter of U.S. patent application Ser. No. 60/079,953, is depicted in FIG. 1a. This surface 10, which may also be referred to as a Frequency Selective Surface (FSS), includes an array of metal elements 12 arranged above a flat metal ground plane 14. The size of each element 12 is much less than the operating wavelength of the antenna. The overall thickness of the structure is also much less than the operating wavelength. The presence of the elements 12 has the effect of changing the boundary condition at the surface, so that it appears as an artificial magnetic conductor, rather than an electric conductor. It has this property over a band gap ranging from a few percent to nearly an octave, depending on the thickness of the structure with respect to the operating wavelength (see FIG. 1c). A Hi-Z surface 10 can be made in various forms, including a multi-layer structure with overlapping capacitor plates. Preferably the Hi-Z structure is formed on a printed circuit board insulating substrate 16 (omitted in FIG. 1a for clarity purposes) with the elements 12 formed on one major surface thereof and the ground plane 14 formed on the other major surface thereof. Elements 12 are preferably electrically coupled to the ground plane 14 by means of conductive vias 18, which vias 18 may be formed by plating through holes formed in the printed circuit board 16. Capacitive loading allows the resonance frequency to be lowered for a given thickness. Operating frequencies ranging from hundreds of megahertz to tens of gigahertz have been demonstrated using a variety of geometries of Hi-Z surfaces. The shapes of elements 12, in plan view, can be square, hexagonal (as shown by FIG. 1a) or any other convenient, repeating geometric shape.
A prior art waveguide fed, aperture-coupled slot or patch antenna is depicted in a side elevational view by FIG. 1d. The patch antenna element 8 is disposed over a back plane 14 which has an opening or slot 9 therein which is directly coupled to the walls of a waveguide 22. These antennas are flat, but they also tend to have high Qs. That is, an acceptable impedance match between the waveguide 22 and the antenna 8 can only be achieved over a rather narrow bandwidth without the use of wideband impedance matching networks. FIG. 1e is a chart showing the simulated results for an antenna of the type shown in FIG. 1d over the frequency range of 11-16 Ghz (plot “A”). The high Q nature of this antenna is plainly evident. Patch antennas are also rather large (they have a physical size of about ½λ for the frequencies of interest), which often makes it difficult to arrange an array of such antennas in a confined space.
There are other techniques well known in the prior art for coupling a waveguide to an antenna structure. However, these prior art structure are not flat. Rather, they have profiles which project in a direction away from the waveguide (in the direction of arrow A in FIG. 1d). Thus, they have profiles, in side elevation view which makes them difficult for use on surfaces which should be either flat or moderated contoured, such a the surface of an aircraft or a land vehicle. In the automotive market, antennas which project from the surface of the vehicle are considered to be rather unsightly. So antennas which are flat (or which can be contoured if need be) are needed. Additionally, there is a need for a technique for coupling a waveguide to an antenna structure which is flat (and preferably which can be contoured when needed) with an acceptable impedance match over a relatively wide frequency band.